The goal of our research is to understand the process by which simian virus 40 transforms cells in culture and causes tumors in animals. To this end, we have constructed and characterized an SV4O mutant, T147D, that lacks many of the SV4O viral replication functions, but which is still able to transform. Because of these properties, T147D should be useful in identifying SV4O functions essential for transformation. Oncogenic transformation involves a number of alterations in cell structure, function, and metabolism, and it is unlikely that the SV4O oncoprotein, T antigen, could cause all these changes directly. It is more probable that T antigen acts indirectly by modulating the expression of key cellular genes, and that this altered pattern of gene expression results, at least in part, in the transformed phenotype. For this reason, it would be of great interest to identify the cellular genes whose expression is modulated by SV4O transformation. We have used subtractive hybridization to identify a cellular gene, D46, that is up-regulated in T147D transformants and in a variety of other oncogene-expressing cells. We propose to characterize this gene further in order to elucidate its role in SV4O-mediated transformation, and to use subtractive hybridization and mRNA differential display to identify other cellular genes that also may be important for transformation.